Seismic velocities from the Yaquina forearc basin off Peru: evidence for free gas within the gas hydrate stability zone

2005 ◽  
Vol 94 (3) ◽  
pp. 420-432 ◽  
Author(s):  
Gesa L. Netzeband ◽  
Christian P. Hübscher ◽  
Dirk Gajewski ◽  
Jan W. G. Grobys ◽  
Jörg Bialas
Keyword(s):  
Gas Hydrate ◽  
Seismic Velocities ◽  
Stability Zone ◽  
Forearc Basin ◽  
Free Gas ◽  
Hydrate Stability

scholarly journals Thermal State of the Blake Ridge Gas Hydrate Stability Zone (GHSZ)—Insights on Gas Hydrate Dynamics from a New Multi-Phase Numerical Model

Energies ◽  
2019 ◽  
Vol 12 (17) ◽  
pp. 3403 ◽  
Author(s):  
Burwicz ◽  
Rüpke
Keyword(s):  
Numerical Model ◽  
Gas Hydrate ◽  
Methane Flux ◽  
Stability Zone ◽  
Free Gas ◽  
Blake Ridge ◽  
Geochemical Profiles ◽  
Multi Phase ◽  
Hydrate Stability

Marine sediments of the Blake Ridge province exhibit clearly defined geophysical indications for the presence of gas hydrates and a free gas phase. Despite being one of the world’s best-studied gas hydrate provinces and having been drilled during Ocean Drilling Program (ODP) Leg 164, discrepancies between previous model predictions and reported chemical profiles as well as hydrate concentrations result in uncertainty regarding methane sources and a possible co-existence between hydrates and free gas near the base of the gas hydrate stability zone (GHSZ). Here, by using a new multi-phase finite element (FE) numerical model, we investigate different scenarios of gas hydrate formation from both single and mixed methane sources (in-situ biogenic formation and a deep methane flux). Moreover, we explore the evolution of the GHSZ base for the past 10 Myr using reconstructed sedimentation rates and non-steady-state P-T solutions. We conclude that (1) the present-day base of the GHSZ predicted by our model is located at the depth of ~450 mbsf, thereby resolving a previously reported inconsistency between the location of the BSR at ODP Site 997 and the theoretical base of the GHSZ in the Blake Ridge region, (2) a single in-situ methane source results in a good fit between the simulated and measured geochemical profiles including the anaerobic oxidation of methane (AOM) zone, and (3) previously suggested 4 vol.%–7 vol.% gas hydrate concentrations would require a deep methane flux of ~170 mM (corresponds to the mass of methane flux of 1.6 × 10−11 kg s−1 m−2) in addition to methane generated in-situ by organic carbon (POC) degradation at the cost of deteriorating the fit between observed and modelled geochemical profiles.

Fluid dynamics at the base of hydrate-bearing sediments at the Vestnesa Ridge inferred from 5 years of high-resolution 4D seismic surveying

2020 ◽  
Author(s):  
Malin Waage ◽  
Stefan Bünz ◽  
Kate Waghorn ◽  
Sunny Singhorha ◽  
Pavel Serov
Keyword(s):  
High Resolution ◽  
Gas Hydrate ◽  
Seismic Data ◽  
Time Lapse ◽  
Stability Zone ◽  
Seismic Surveys ◽  
Free Gas ◽  
4D Seismic ◽  
Hydrate Bearing Sediments ◽  
Hydrate Stability

<p>The transition from gas hydrate to gas-bearing sediments at the base of the hydrate stability zone (BHSZ) is commonly identified on seismic data as a bottom-simulating reflection (BSR). At this boundary, phase transitions driven by thermal effects, pressure alternations, and gas and water flux exist. Sedimentation, erosion, subsidence, uplift, variations in bottom water temperature or heat flow cause changes in marine gas hydrate stability leading to expansion or reduction of gas hydrate accumulations and associated free gas accumulations. Pressure build-up in gas accumulations trapped beneath the hydrate layer may eventually lead to fracturing of hydrate-bearing sediments that enables advection of fluids into the hydrate layer and potentially seabed seepage. Depletion of gas along zones of weakness creates hydraulic gradients in the free gas zone where gas is forced to migrate along the lower hydrate boundary towards these weakness zones. However, due to lack of “real time” data, the magnitude and timescales of processes at the gas hydrate – gas contact zone remains largely unknown. Here we show results of high resolution 4D seismic surveys at a prominent Arctic gas hydrate accumulation – Vestnesa ridge - capturing dynamics of the gas hydrate and free gas accumulations over 5 years. The 4D time-lapse seismic method has the potential to identify and monitor fluid movement in the subsurface over certain time intervals. Although conventional 4D seismic has a long history of application to monitor fluid changes in petroleum reservoirs, high-resolution seismic data (20-300 Hz) as a tool for 4D fluid monitoring of natural geological processes has been recently identified.<br><br>Our 4D data set consists of four high-resolution P-Cable 3D seismic surveys acquired between 2012 and 2017 in the eastern segment of Vestnesa Ridge. Vestnesa Ridge has an active fluid and gas hydrate system in a contourite drift setting near the Knipovich Ridge offshore W-Svalbard. Large gas flares, ~800 m tall rise from seafloor pockmarks (~700 m diameter) at the ridge axis. Beneath the pockmarks, gas chimneys pierce the hydrate stability zone, and a strong, widespread BSR occurs at depth of 160-180 m bsf. 4D seismic datasets reveal changes in subsurface fluid distribution near the BHSZ on Vestnesa Ridge. In particular, the amplitude along the BSR reflection appears to change across surveys. Disappearance of bright reflections suggest that gas-rich fluids have escaped the free gas zone and possibly migrated into the hydrate stability zone and contributed to a gas hydrate accumulation, or alternatively, migrated laterally along the BSR. Appearance of bright reflection might also indicate lateral migration, ongoing microbial or thermogenic gas supply or be related to other phase transitions. We document that faults, chimneys and lithology constrain these anomalies imposing yet another control on vertical and lateral gas migration and accumulation. These time-lapse differences suggest that (1) we can resolve fluid changes on a year-year timescale in this natural seepage system using high-resolution P-Cable data and (2) that fluids accumulate at, migrate to and migrate from the BHSZ over the same time scale.</p>

Characterization of gas accumulation at a venting gas hydrate system in Shenhu area, south china sea

2021 ◽  
pp. 1-45
Author(s):  
JInqiang Liang ◽  
Zijian Zhang ◽  
Jingan Lu ◽  
Guo Yiqun ◽  
Zhibin Sha ◽  
...  
Keyword(s):  
South China Sea ◽  
Wave Velocity ◽  
South China ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Stability Zone ◽  
Shenhu Area ◽  
Free Gas ◽  
Seismic Amplitude ◽  
Hydrate Stability

Bottom-simulating reflections (BSR) in seismic data have been widely accepted to indicate the base of methane gas hydrate stability zone (MGHSZ) and free gas was thought to exist only below it. However, real geologic systems are far more complex. Here, we presented the results of three-dimensional seismic, logging while drilling (LWD), in situ and coring measurements at a venting gas hydrate system in the Shenhu area of the South China Sea. Our studies reveal that free gas has migrated upward through the thermogenic gas hydrate stability zone (TGHSZ) into the MGHSZ and become a part of the gas hydrate system. Seismic amplitude anomalies and core results suggest the presence of free gas above the base of MHSZ at 165 mbsf and the presence of thermogenic gas hydrates below it in the well SC-W01. Analyses of P-wave velocity, S-wave velocity, density, and porosity logs reveal free gas occurs above and below the MGHSZ as well. Integrating log and core analysis with seismic interpretation suggests that the variation in seismic amplitude within chaotic zone is associated with variable gas saturations, and a large amount of methane and thermogenic gases accumulate near the complex BSRs. We propose that relative permeability likely plays a significant role in the free gas distribution and formation of gas hydrates within a venting gas hydrate system, while the effect of dissolved-gas short migration is not ignored. Our results have important implications for understanding the accumulation and distribution of gas hydrates and free gas in the venting gas hydrate system and seeps at the seafloor.

Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized core

2004 ◽  
Vol 222 (3-4) ◽  
pp. 829-843 ◽  
Author(s):  
Alexei V. Milkov ◽  
Gerald R. Dickens ◽  
George E. Claypool ◽  
Young-Joo Lee ◽  
Walter S. Borowski ◽  
...  
Keyword(s):  
Gas Hydrate ◽  
Stability Zone ◽  
Free Gas ◽  
Hydrate Ridge ◽  
Hydrate Stability

Evaluation of gas hydrate-bearing sediments below the conventional bottom-simulating reflection on the northern slope of the South China Sea

2017 ◽  
Vol 5 (3) ◽  
pp. SM61-SM74 ◽  
Author(s):  
Jinqiang Liang ◽  
Zijian Zhang ◽  
Pibo Su ◽  
Zhibin Sha ◽  
Shengxiong Yang
Keyword(s):  
South China Sea ◽  
South China ◽  
Gas Hydrate ◽  
The South China Sea ◽  
Northern Slope ◽  
Stability Zone ◽  
Shenhu Area ◽  
Free Gas ◽  
Bottom Simulating Reflection ◽  
Hydrate Stability

The continuous bottom-simulating reflection (BSR) is commonly considered to mark the base of gas hydrate stability zone. Below this depth, gas hydrate gives away to free gas or water filling with pore spaces of sediments. We integrated and analyzed seismic data collected in 2008, and logging-while-drilling (LWD) data and coring results acquired by the Fugro Voyager in 2015 in the Shenhu area on the northern slope of the South China Sea. Based on seismic and well-log correlation, a BSR with typical characteristics of gas hydrates and free gas was identified at 237 m, below the mudline (BML). However, LWD data reveal a 63 m thick hydrate layer from 205 to 268 m BML. Increases in resistivity and velocity at 262 m BML indicate that gas hydrate is likely presented below the BSR. The observed pore-water freshening with depth and infrared image of core samples are consistent with geophysical interpretation. Seismic and well interpretations reveal continuous, discontinuous, and pluming BSRs in the Shenhu area. The continuous BSR indicates the base of the methane gas hydrate stability zone, and structure II gas hydrate is likely presented below the BSR. Deep thermogenic fluid locally entrapped within shallow-buried sediments may reinforce gas-hydrate accumulations near the discontinuous and pluming BSRs. We conclude that seismic responses of structure II gas hydrate can be distinct from structure I gas hydrate. Understanding the seismic characterizations of structures I and II will help in the evaluation of gas-hydrate reservoirs and inferring the presence of deep thermogenic reservoirs.

Gas hydrates at the Storegga Slide: Constraints from an analysis of multicomponent, wide-angle seismic data

Geophysics ◽  
2005 ◽  
Vol 70 (5) ◽  
pp. B19-B34 ◽  
Author(s):  
Stefan Bünz ◽  
Jürgen Mienert ◽  
Maarten Vanneste ◽  
Karin Andreassen
Keyword(s):  
Gas Hydrates ◽  
Gas Hydrate ◽  
Seismic Data ◽  
Wide Angle ◽  
Stability Zone ◽  
Gas Distribution ◽  
Free Gas ◽  
Storegga Slide ◽  
Gas Bearing ◽  
Hydrate Stability

Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km[Formula: see text], outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.

Time Constraints on Seepage Through Fractured Regions on the Vestnesa Ridge off the W-Svalbard Coast

2021 ◽  
Author(s):  
Hariharan Ramachandran ◽  
Andreia Plaza-Faverola ◽  
Hugh Daigle ◽  
Stefan Buenz
Keyword(s):  
Fluid Flow ◽  
Effective Stress ◽  
Gas Hydrate ◽  
Fluid Pressure ◽  
Fracture Network ◽  
The Arctic ◽  
Stability Zone ◽  
Methane Seepage ◽  
Carbonate Formation ◽  
Hydrate Stability

<p>Evidences of subsurface fluid flow-driven fractures (from seismic interpretation) are quite common at Vestnesa Ridge (around 79ºN in the Arctic Ocean), W-Svalbard margin. Ultimately, the fractured systems have led to the formation of pockmarks on the seafloor. At present day, the eastern segment of the ridge has active pockmarks with continuous methane seep observations in sonar data. The pockmarks in the western segment are considered inactive or to seep at a rate that is harder to identify. The ridge is at ~1200m water depth with the base of the gas hydrate stability zone (GHSZ) at ~200m below the seafloor. Considerable free gas zone is present below the hydrates. Besides the obvious concern of amount and rates of historic methane seeping into the ocean biosphere and its associated effects, significant gaps exist in the ability to model the processes of flow of methane through this faulted and fractured region. Our aim is to highlight the interactions between physical flow, geomechanics and geological control processes that govern the rates and timing of methane seepage.</p><p>For this purpose, we performed numerical fluid flow simulations. We integrate fundamental mass and component conservation equations with a phase equilibrium approach accounting for hydrate phase boundary effects to simulate the transport of gas from the base of the GHSZ through rock matrix and interconnected fractures until the seafloor. The relation between effective stress and fluid pressure is considered and fractures are activated once the effective stress exceeds the tensile limit. We use field data (seismic, oedometer tests on calypso cores, pore fluid pressure and temperature) to constrain the range of validity of various flow and geomechanical parameters in the simulation (such as vertical stress, porosity, permeability, saturations).</p><p>Preliminary results indicate fluid overpressure greater than 1.5 MPa is required to initiate fractures at the base of the gas hydrate stability zone for the investigated system. Focused fluid flow occurs through the narrow fracture networks and the gas reaches the seafloor within 1 day. The surrounding regions near the fracture network exhibit slower seepage towards the seafloor, but over a wider area. Advective flux through the less fractured surrounding regions, reaches the seafloor within 15 years and a diffusive flux reaches within 1200 years. These times are controlled by the permeability of the sediments and are retarded further due to considerable hydrate/carbonate formation during vertical migration. Next course of action includes constraining the methane availability at the base of the GHSZ and estimating its impact on seepage behavior.</p>

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